1. Field of the Invention
The present invention relates to a catalytic convertor for an automobile, and in particular, to a carrier for supporting a catalytic material thereon. More particularly, the present invention relates to such a catalyst carrier made or metal sheets firmly joined to each other by an improved method of connecting same.
2. Description of the Related Art
A honeycomb shaped body made of ceramic material based on cordierite is currently widely used as a carrier of a catalytic material for a catalytic convertor for purifying exhaust gases discharged from an internal combustion engine of an automobile. This type of catalyst carrier, however, has a drawback in that it is very easily damaged when subjected to a mechanical shock, since it is made from a brittle ceramic material. Accordingly, a catalyst carrier has been proposed wherein a metal material is employed in place of the ceramic material.
To produce such a metal material carrier, first a sheet of a metal material such as stainless steel or an oxidation resistant high alloy steel, having a thickness of about 40 .mu.m is obtained, and then a continuous length of a plain sheet is wound around a core while a continuous length of a corrugated sheet is fixed to the surface of the plain sheet, to thus obtain a honeycomb structure. Alternatively, a certain length of a plain sheet and a certain length of a corrugated sheet are laminated together to obtain a honeycomb structure. The obtained honeycomb structure is housed in an outer casing made of stainless steel or an oxidation resistant high alloy steel having a thickness of 1 to 2 mm. Note, during the formation of the honeycomb structure, a brazing material is applied to the sheets at portions at which the sheets are in contact with each other. Finally, the honeycomb body housed in the casing is heated in a vacuum to carry out the brazing process needed for connecting the honeycomb structure to the inner wall of the outer casing, and to connect the plain sheets and corrugated sheets to each other at portions at which these sheets are in contact with each other.
This brazing connection process makes the honeycomb structure a rigid structure, since the sheets forming the honeycomb structure are connected to each other at points along the entire length of the layers of the honeycomb structure. Nevertheless, when this structure is applied to a catalytic convertor mounted in an exhaust pipe of an automobile, stress is apt to be concentrated on the honeycomb structure at points near the outer casing, which causes a separation or cracking of the portions of the sheets in contact with one another, because the carrier is subjected to a strong thermal stress caused not only by vibration applied thereto when the vehicle is moving but also by a thermal cycle generated by a repetition of rapid heating and cooling thereof. Such a thermal stress is generated mainly by a difference in the degree of the thermal expansion of the outer casing and that of the honeycomb structure at the outer portion thereof. Therefore, to obviate this difficulty, a partially connected structure is proposed, wherein the sheets are connected at a limited number of points along the entire length of the layers of the honeycomb structure, to thus obtain a less rigid construction. This improved structure, however, has a drawback in that the process for obtaining such a structure is extremely complicated, and thus this proposal is not practical when trying to overcome the problems of the prior art. Furthermore, the prior art method is expensive, since the brazing material, and process for carrying out the brazing, are costly.
In view of the above, various methods have been proposed in order to obviate the above-mentioned difficulty. In such an improved method, in place of the brazing process, an improved means is used for connecting the layers, using a high density energy source such as a laser beam or electron beam, or an electric resistance welding process. For example, Japanese Unexamined Patent Publication No. 62-71547 discloses a method wherein a spot welding is first carried out to connect the plain sheet and the corrugated sheet, which are positioned facing each other, and then the thus-connected sheets are spirally wound on a core to thereby obtain a honeycomb structure. In this construction, rigid connections between the plain sheet and the corrugated sheet are obtained only in each of the layers constructing a honeycomb structure, and therefore, a rigid connection of the sheets is not located between the layers constructing the honeycomb structure. As a result, an axial force, applied to the honeycomb structure when placed in a flow of the exhaust gas to be treated for removal of toxic components, is created and displaces the layers in a telescopic manner. Another prior art, Japanese Unexamined Patent Publication No 64-40180, discloses an electric resistance welding process carried out by electrode units arranged in parallel, each unit comprising two electrodes arranged outside of the honeycomb structure. This method, however, has a drawback in that the welding can be carried out only on a limited area of the circumference of the core, the number of electrodes that can be arranged is limited, the welding process and winding process must be carried out alternatively and separately, and the welding can be done only intermittently, which prolongs the production time, and thus lowers the production efficiency.